1. Field of the Invention
The present invention describes a cascode amplifier.
2. State of the Art
Amplifiers of the cascode type are often used to obtain high gain and high output impedance and present good characteristics of linearity of the output signal. A typical configuration of a cascode circuit stage, shown in FIG. 1, is constituted by a common source MOS transistor M1 and by a common gate MOS transistor having the source terminal in common with the drain terminal of the transistor M1. The input terminal IN is the gate terminal of the transistor M1 while the output terminal OUT is the drain terminal of the transistor M2. The polarisation voltages V1–V3 are connected respectively with the drain terminal of the transistor M2, with the gate terminal of the transistor M2 and with the gate terminal of the transistor M1. Normally a minimisation of the Miller effect on the capacitance present between the drain and the gate terminals of the transistor M1 is obtained with this type of amplifier considering that the voltage on the drain terminal of the transistor M1 is set by the voltage on the source terminal of the transistor M2. Nevertheless, above all in the case of wide variations of voltage on the input terminal and in the case in which the equivalent resistance on the source terminal of the transistor M2 is high, there is an increase of the Miller effect; this causes high distortion of the output signal from the amplifier.
The increase of the Miller effect is still more remarkable in the case in which the cascode amplifier has to have a wide field of variation of the voltage. In that case in fact the cascode amplifier presents a circuit structure with feedback by means of a passive element.
A circuit type of a cascode amplifier with feedback used is shown in FIG. 2 where a MOS transistor M11 with common gate is positioned in the place of the MOS transistor M1 having as input terminal the source terminal; a resistance Rf is placed between the gate terminal of the transistor M11 and the output terminal OUT of the cascode amplifier.